This invention relates to an improved process for preparing ethanolamines by reacting ethylene oxide with ammonia. From another viewpoint, the invention relates to a process for effectively recovering the unreacted ammonia in the reaction of ethylene oxide (EO) with ammonia (NH.sub.3).
Ethanolamines find a wide range of applicants as detergents, emulsifiers, gas absorbents, corrosion inhibitors, gloss-imparting agents, polishes, textile treating assistants, and raw materials for herbicides, pharmaceuticals, etc.
They can be produced by the reaction of ethylene halohydrins or ethylene oxide with ammonia, but the current commercial-scale production of ethanolamines almost entirely relies on the reaction of ethylene oxide with ammonia.
The reaction of ethylene oxide addition to ammonia not only yields monoethanolamine (MEA), but also diethanolamine (DEA) and triethanolamine (TEA). The ratios of the ethanolamines in the reaction product is determined by the mol ratio of ammonia to ethylene oxide. For example, to obtain a reaction product containing a major proportion of MEA, a large excess of ammonia should be reacted with ethylene oxide. This procedure requires a heating source, for example a large quantity of steam, to recover the excess of ammonia after the reaction.
The addition reaction between ethylene oxide and ammonia is an exothermic reaction. In known processes using reactors with shell and tubes, it is the common practice to cool the outside of a reaction zone with cooling water, etc. so as to restrict the reaction temperature to 50.degree. to 150.degree. C. It is also known to react ethylene oxide and ammonia adiabatically, and desorb the ammonia by releasing the elevated pressure. Since the inside of the reactor attains high temperatures and pressures, a large-scale apparatus is required, and the resulting ethanolamines, especially TEA, tend to be colored. In this process, the equipment needed becomes larger, the larger the ratio of NH.sub.3 /EO. Water is employed both as a catalyst and an absorbent in the reaction, the latter facilitating recycle of the excess ammonia.
The present improved process, by employing a high pressure flash distillation and liquification of part of the NH.sub.3 avoids the need for ever larger absorbers or compressors as the ratio of NH.sub.3 /EO increases. This also reduces the severity of the process conditions necessary to recycle the ammonia when the ratio of H.sub.2 O/NH.sub.3 fed to the reactor is limited (reduced) for economic reasons. The direct liquefaction of NH.sub.3 is also a convenient and economical method to prepare NH.sub.3 for recycle in the anhydrous reaction of NH.sub.3 and EO.